It's an eco-tour, so...something made out of hemp? Recycled tennis shoes? A paper bag? Eh, probably something more practical and a little more rugged. Buy good quality, make it last for years, thats one way to be eco-sensitive.
The Truist Pack
What you get kinda depends on whether you'd rather carry something or roll it along behind you. If you prefer to roll it, then a great way to eco-hoard your dollars is with REI's Wheely Beast 28-Inch Rolling Duffel ($149). With 6,000 cubic inches of space, it'll swallow all the gear you might need on a ten-day tour of Costa Rica, or just about any other trip. It's basically just a big bag, though, so organizing your stuff will take a little thought; I suggest you try to pack things in order of when you expect to need them.
If you prefer to carry your stuff, Eagle Creek's Truist Pack 65L ($230) is just the ticket. It has 3,950 cubic inches of packable space, so it will force you to be a little selective about what you take. But that's still plenty of room, and the Truist comes with a comfortable suspension for heavy loads.
But waitwhat if you want a pack you can roll OR put on your back? Well, several companies make those. Osprey's Meridian 28 ($319) has loads of space (4,600 cubic inches), wheels and a handle, and shoulder straps that pop out for those times when lugging a pack is easiest. It's not really a pack for schlepping over hill and dale, but it's fine for jaunts through the airport or around town. It even has a little daypack you can zip off and carry, just to ensure everyone knows you are a tourista.